The program is addressed to the development of non-invasive methods for evaluation of nutritional status and body composition in pediatric patients. We will examine the kinetics of 13CO2 exhalation in breath after bolus administration of NaH13CO3 and then calculate significant parameters from the time course of tracer disappearance. After a fast exponential distribution, the disappearance of 13CO2 from breath is biexponential over a three-hour period, in keeping with the concept that tracer is cleared first more rapidly from perfused, metabolically active viscera and then from the more slowly perfused muscle mass. Area and moment analysis of the decay coefficients and exponents is expected to quantify these compartments in the fasting state in such a way that interindividual variations in endogeneous metabolic rate and in body composition (viscera vs muscle) may be measured quantitatively. Thus, the goals are to develop and validate a breath test for nutritional assessment, that can be used either by real-time monitoring with a metabolic cart for nonradioactive carbon isotope detection or by interval sampling procedures amenable to work up in the clinical laboratory. If successful, the approach would represent an innovation over existing methods by combining the measurement of caloric requirements in the same test that characterizes body composition.